The recording channel of a flexible media disk file presents unique detection requirements which are generally not found in disk files employing rigid disks. In flexible media disk files and in magnetic tape drives, where the head is not in contact with the medium and data is stored in run-length limited NRZI code, a bandwidth limited version of the derivative of the write waveform is provided on reading. In such an environment, the detection of the read back system must be amplitude sensitive because the raw data signal provided by the magnetic transducer has three discrete levels, i.e., positive, zero, and negative. A binary amplitude detection technique, therefore, cannot be used. In addition, since the magnetic coating of flexible media is relatively more susceptible to drop-outs than rigid media, relatively large variations in amplitude are encountered. In addition, in disk files the amplitude of the read back signal will vary as a function of the linear velocity of the medium past the transducer which varies in a disk file as a function of the radial position of the head relative to the disk. Further, when the data has been recorded in a run-length limited code, the detector must be able to detect while in the presence of noise signals caused by overwriting of data on the track and, in disk files, the slight mispositioning of the moving head relative to the data track. It is also desirable in today's technology to implement the read back system in simple analog integrated circuits, which requires the signal processing techniques to be relatively simple.